Respiratory therapy systems using mechanical ventilation for moving gas into a patient's lungs commonly incorporate a humidifier along the respiratory circuit in order to humidify the respiratory gas directed to the patient. Examples of such humidifiers are disclosed in U.S. Pat. Nos., 4,110,419, 4,172,105, 4,195,044, 4,500,480 and 4,674,494, the disclosures of which are incorporated herein by reference. Such humidifiers utilize a cartridge-type humidifier positioned in a heater, with water supplied from a pre-filled sterile water reservoir. Hudson Respiratory Care Inc. markets such a system as CONCHA.RTM.. A heater used for heating the cartridge humidifier is described in U.S. Pat. No. 4,564,748, the description of which is incorporated herein by reference. Such an apparatus provides heated humidification to be delivered to the patient during mechanical ventilation therapy.
In mechanical ventilation, a ventilator forces respiratory gas along the ventilation circuit tubing into the patient's lungs. When mechanical ventilation therapy is prescribed to children, infants and neonates it is desirable to limit the amount of pressure delivered to the patient so that injury to the lung may be avoided. The pressure required to deliver the prescribed amount of gas to the patient is dependent upon the resistance and compliance characteristics of the patient's lungs, as well as the compliance or volume of gas in the respiratory circuit between the ventilator and the patient. Thus, the compressible volume of gas inside the humidifier chamber during the inspiratory phase is an important factor in the delivery of gas to the patient's lungs at the lowest pressure to properly ventilate the patient Where the volume of gas inside the humidifier chamber is substantial, the greater the compliance of the system is, and the more ventilator pressure required to deliver the appropriate amount of gas to the patient so that the blood carbon dioxide level may be appropriately maintained. Although for most adults, high mechanical ventilation pressures are often not of such concern, for pediatric, and particularly neonatal ventilation, relatively low pressure ventilation is of substantial criticality, with lung pressures of under 30 cm H.sub.2 O being desirable to prevent lung injury. In order to assure such low-pressure performance, it is desired to effectively reduce the compliance of the humidifier used in the ventilator circuit. It is to such a reduced and improved performance humidifier that the present invention is directed.